1. Field of the Invention
This invention relates to navigation systems and more particularly to a vehicle navigation system and method for displaying, to a user, road sign information for route guidance.
2. Description of the Prior art
Vehicle navigation systems are well known; see e.g. Yamada et al., U.S. Pat. No. 4,926,336; Link et al., U.S. Pat. No. 5,270,937; and Ichikawa, U.S. Pat. No. 5,410,485. Such navigation systems typically include two major functions. One function is route guidance which determines the optimum route to be followed from a starting point to a destination, typically along a network of roads. The other function is displaying to the user of the system (typically the driver of the vehicle), both the current location and the route to be followed. This is usually in the form of a map displayed on a small display screen installed in the vehicle. Such systems are well known and not described herein in further detail.
As is well known, typically in such systems a database is provided of the road network. This database is all of the roads for instance in one state or in some other fairly large geographical area. This representation of the road network is sometimes referred to as a digitized map database. It includes for each road the road name, and divides each road into one or more road segments, where a road segment extends from one road node to another. A road node is typically either an intersection with another road or an intersection of a road with some arbitrary boundary, e.g. a state boundary and has an associated latitude and longitude.
In addition to providing the actual road information for use by the system, a typical digitized map database also includes road sign information. That is, when the database is compiled, i.e., by a survey of the roads, the surveyor also enters into the database the textual contents of at least some of the actual road signs encountered along each road segment. This actual road sign information is used in the prior art for route guidance, especially on roads such as freeways or equivalents (multi-lane limited access roads). When the system is providing route guidance such as from a freeway exiting onto a surface street, road sign information, referred to as "sign text" is displayed on the display screen to the user. This sign text allows the driver to recognize the actual road sign information at a bifurcation in the road, i.e. a node, and more easily decide which road to follow. Thus this display of sign text enhances the route guidance function.
An example of this is shown in FIG. 1 where the guided route indicated by the dotted line is from the current vehicle location on freeway C to the Harbor Freeway 110 South. This route, involving a connection from one freeway to another, requires driving along the ramp connecting the two freeways. In the context of this application a "ramp" connects roads that do not intersect at grade. When a ramp intersects with another road, the ramp ends. Hence a ramp is e.g. a route entering a freeway (or equivalent) from a surface street; a route exiting from a freeway (or equivalent) onto an ordinary street; a route connecting one freeway (or equivalent) with another freeway (or equivalent); and also a route connecting two non-freeways (or equivalents), for instance a ramp connecting the upper and lower roadways at an airport. A ramp, like any other route, includes at least one road segment. These ramp road segments connect at decision (node) points. For instance as shown in FIG. 1, the ramp segment 1 begins at the first decision point DPT1 and ends at a second decision point DPT2. A decision point is a node at which the navigation system determines (or the database indicates) that route guidance is required. The sign text associated with the first decision point, i.e., the sign text stored in the database associated with the first decision point DPT1, may contain information relating successive decision points along the guided route. If a decision point lies between two road segments (e.g. road segment 1 and road segment 2), and the vehicle is traveling from segment 1 to segment 2, the sign text associated with that decision point is attached in the database to road segment 2.
As shown in FIG. 1, the sign text data stored in the database and associated with the first decision point DPT1 is 110 Harbor Freeway/San Pedro/Los Angeles. The sign text data stored in the database and associated with the second decision point DPT2 is 110 South Harbor Freeway/San Pedro. FIG. 1 also shows the text of the actual physical road signs of each of these decision points.
Hence the sign text data is an edited version of the actual text of the road sign and is often not the identical words nor arranged identically. Typically the sign text data is somewhat edited to make it meaningful for use in the database. This editing is a part of the compilation of the database. It is to be understood that typically the database is produced and sold separately from the actual navigation system since such a navigation system must be useful anywhere e.g. in the United States, and typically a user would not purchase a database covering the entire United States but would only purchase a database for a part of the United States. Hence the databases are usually sold in the form of e.g. a CD-ROM containing the database needed by a particular user for a particular geographical area.
In the prior art at the guidance point GPT1, which is prior to the associated decision point DPT1, the system displays on its screen the sign text 110 Harbor Freeway/San Pedro/Los Angeles. Later on at guidance point GPT2 along the guided route, guidance point GPT2 being somewhat before the associated decision point DPT2, the system displays the sign text 110 South Harbor Freeway/San Pedro. It can be seen that the guidance points do not correspond to any actual physical entity on the road network, but instead are only a part of the database. A guidance point is a point at which an instruction is given at a certain distance before the associated decision point. The distance depends on the type of road segment on which the vehicle is currently traveling. Of course, it is necessary for the guidance to be given prior to the driver reaching the actual decision point, since otherwise the guidance would be of little or no use. The distance between the decision point and the associated guidance point is somewhat arbitrary but is typically set such that the driver has sufficient time to look at the display and react, and hence represents a travel distance at a typical driving speed along the particular road segment in question of e.g. 30 seconds.
Thus at the first guidance point GPT1 the sign text is essentially the same as the text of the actual road sign. At the second guidance point GPT2 the sign text data corresponds to a portion of the actual text of the road sign at the second decision point DPT2.
While of some use, this method has the disadvantage that the particular sign text associated with a decision point may contain too much information to fit on a limited size display, and also may confuse and distract the user. It is to be understood that the display typically is either a very small CRT or a small liquid crystal display, measuring only a few inches wide. In addition to displaying the sign text, typically the display at the same time displays a representation of a map of the roads at the current vehicle position and along the guided route.
Hence while the prior art display method is accurate in the sense that it depicts either actual road signs or gives guidance, the guidance may be confusing and hence of limited value. Since of course the driver can only give very limited attention to the navigation system display in order to drive safely, even a relatively minor distraction or a partially confusing representation of information is considered to be very undesirable in such systems.